POS0304 IN-SILICO MODELLING OF THE SACROILIAC JOINTS DEMONSTRATES INFLUENCE OF JOINT FORM VARIATION ON MECHANICAL STRESS
Authors: M. Heyland, D. Maikath, P. Damm, K. G. Hermann, K. Ziegeler
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> Recent studies have uncovered an association between atypical joint form morphology and mechanical disease of the sacroiliac joints <sup>[1]</sup> and sclerosis in healthy controls <sup>[2]</sup>. A possible explanation for the effect is a change in load distribution in atypical joint forms and thus increased mechanical stress in affected patients. <h3>Objectives</h3> To investigate the effect of joint morphology on joint stress, using finite element models (FEM). <h3>Methods</h3> FE models were computed using dedicated software (Amira Software, Zuse Institute Berlin and Thermo Fisher Scientific, 2021), from CT scans of five patients without disease of the sacroiliac joints from a retrospective patient cohort<sup>[2]</sup>. Selected were patients with known anatomical variants (accessory joint, crescent-shaped ilium and intra-articular joint form variant; all female individuals) as well as a typical male and a typical female joint, based on radiological assessment. The models included both information on bone elasticity, derived from bone density on CT and stiffness of ligaments and muscles from the literature (visualization in Figure 1). Lastly, loading conditions during bipedal walking were simulated in all models, drawing from in-vivo data. Mean von Mises stress as well as percentage of joint surface above critical stress value (defined as 7 MPa) during y-axis loading (e.g. standing upright) and bipedal walking were compared between different joint morphologies. <h3>Results</h3> Table 1 provides a summary of the results. In axial loading (=y-axis stress) highest median stress and 99<sup>th</sup> percentile stress was observed in the crescent-shaped ilium with 12.5 Mpa and 28.8 Mpa respectively, while the iliosacral complex exhibited the lowest median stress (7.0 MPa) and the second highest 99<sup>th</sup> percentile value (25.1 MPa), indicating a less even distribution of stress. For bipedal walking the highest stress was also observed in the crescent shaped ilium with 64.2% of joint surface showing more than 7 Mpa during ipsilateral toe off vs. only 22.7% in the iliosacral complex. In all load scenarios, stresses were higher in the typical female joint than in the typical male joint. <h3>Conclusion</h3> Finite element modelling revealed differences in load extent and distribution across different sacroiliac joint morphologies, strengthening the evidence, that morphology may play a role in mechanical joint stress. Further studies with larger numbers of computed models per joint form are needed to validate these first exploratory results. <h3>References</h3> [1]Ziegeler K, Kreutzinger V, Proft F, et al. Joint anatomy in axial spondyloarthritis: strong associations between sacroiliac joint form variation and symptomatic disease. <i>Rheumatology (Oxford</i>) 2021 doi: 10.1093/rheumatology/keab318 [published Online First: 2021/04/07] [2]Ziegeler K, Kreutzinger V, Diekhoff T, et al. Impact of age, sex, and joint form on degenerative lesions of the sacroiliac joints on CT in the normal population. <i>Sci Rep</i> 2021;11(1):5903. doi: 10.1038/s41598-021-85303-5 [published Online First: 2021/03/17] <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> Mark Heyland: None declared, Daven Maikath: None declared, Philipp Damm: None declared, Kay-Geert Hermann Speakers bureau: MSD, Novartis, Pfizer, Consultant of: AbbVie, Employee of: BerlinFlame GmbH (founder), Katharina Ziegeler Grant/research support from: ASAS (research grant 2021)